This invention relates to optical fibres and in particular infrared optical fibres and coating methods therefor.
The current high level of interest in mid-infrared transmitting fibres arises primarily from their potential as ultra low attenuation communicating fibres. Among the materials already identified for such use are glasses based on halides and chalcongenides. However, the penalty of attaining increased infrared transmission over that of silicates is a significant reduction in thermal, mechanical and chemical stability. For example, fluorozirconate glasses, which are amongst the prime candidates at present, crystallise spontaneously near 400.degree. C., have low tensile strengths, and in fibre form are rapidly attacked by liquid water and moist air.
To realise the potential of these glasses it will be necessary to include a primary coating which hermetically seals the fibre from ingress of moisture. Metals are particularly attractive from this point of view, and since their expansion coefficients are similar to those of fluoride glasses, minimal contraction stresses are likely to be set up. However, the low thermal stability of these glasses prevents application of most metals from the melt, and other processes such as vapour deposition or sputtering require vacuum or reduced pressure and are incompatible with on-line fibre processings.